1. Field of the Invention
This invention relates to electrostatographic copying and/or printing machines, and more particularly to the maintenance of high image quality in the presence of transient changes in process control parameters.
2. Description of the Prior Art
In electrostatographic machines such as printers and copiers, image contrast, density, and color balance can be adjusted by changing certain process control parameters. Such parameters most frequently include primary voltage V.sub.0, exposure E, and development station electrode bias voltage V.sub.B. Other process control parameters which are less frequently used, but which are effective to control the image contrast, density, and color balance include the concentration of toner in the developer mixture, and the image transfer potential.
Techniques exist for regulating electrostatographic machine process control parameters so as to compensate for long term variations in the electrostatographic process. The phrase "long term" pertains to variations which would affect many successive images, and includes variations caused by such things as changes in toner concentration, wear of the image transfer member, aging of the exposure lamp, and atmospheric conditions. Generally, a test patch or patches are formed and developed on non-image areas of the transfer member. By feedback processes, abnormal toner density readings of the patches result in adjustments to at least some of the process control parameters to return the readings to nominal values.
Other techniques exist for regulating electrostatographic machine process control parameters so as to compensate for short term variations in the electrostatographic process. The phrase "short term" pertains to variations which would affect only one image or just a few successive images, and includes variations caused by such things as short duration electrical transients and differences in the frame-to-frame film thickness of the image transfer member. U.S. Pat. No. 2,956,487 which issued on Oct. 18, 1960 to Giaimo, and U.S. Pat. No. 3,611,982 which issued on Oct. 12, 1971 to Coriale et al show apparatus wherein the post-exposure photoconductor background voltage is sensed, and the voltage level is fed forward to adjust the bias at the development station, thereby maintaining the development bias at a predetermined level in relation to the photoconductor background voltage; normally a desirable goal.
While both feedback, to compensate for long term variations in the electrostatographic process, and feedforward, to compensate for short term variations, work well alone, they tend to cancel out their advantageous effects when used in combination. For example, a feedback adjustment of the primary charger and/or to the exposure control to change the background voltage in an attempt to correct an undesirable change in density would be substantially negated by a feedforward system which detects the change in the background voltage on the photoconductor and adjusts the development bias; resulting in zero change to the density of the developed image.